This invention relates to cathode ray tubes and, more particularly, to luminescent screens for use in such tubes.
The most common cathode ray tubes (CRTs) utilize a powdered phosphor on a carrier as a luminescent screen. These screens have relatively low thermal loadability since heat is insufficiently dissipated from the phosphor grains. As a consequence, during high brightness operation the phosphor has low quantum efficiency and may even be severely damaged. In addition, powdered phosphors exhibit coulombic degradation; that is, quantum efficiency declines due to electron bombardment. This problem is particularly acute in high brightness applications when high electron beam current is used (e.g., in projection CRT applications).
A partial solution to this problem is described in British patent application G.B. No. 2,000,173A which proposes that the luminescent screen be fabricated from a self-supporting monocrystalline body which includes a luminescent layer containing at least one activator. This screen purports to reduce diffuse reflections and increase heat dissipation, thus improving resolution and thermal loadability. Garnet crystal structures with Tb, Tm, Eu, Ce or Nd activators are said to be preferred.
The single crystal nature of the screen, however, gives rise to light trapping inside the monocrystalline layer which has a relatively high refractive index relative to its surroundings. This trapping phenomenon reduces the brightness which would otherwise be obtainable from the screen. However, the brightness obtainable from any luminescent screen, whether a single crystal or powdered material is used, is limited by power saturation of the phosphor; that is, beyond the saturation point, additional increases in electron beam power density do not yield significantly increased brightness. In addition, in certain cases the practical limit to achievable brightness is caused by heating of the phosphor, or by the inability to focus a high current electron beam to the desired spot size. In many applications (e.g., projection CRT.), that practically achievable brightness level is insufficient.
In my copending application Ser. No. 555,167 filed on Nov. 25, 1983 (abandoned in favor of continuation-in-part application Ser. No. 749,928 filed on June 28, 1985), however, light trapping within a phosphor layer is exploited advantageously in a luminescent screen with enhanced brightness. In accordance with one embodiment described in that application, the luminescent screen of a cathode ray tube includes a monoscrystalline or amorphous phosphor layer shaped into an array of elongated, essentially parallel, rod-like elements each having at one end an output face from which light escapes and a reflective coating which covers other surfaces of the element. An electron beam with an oblong cross-section is made incident along the elongated dimension of selected ones of the elements. In this scheme, the resolution is determined by the cross-sectional dimensions of each rod-like element and is not limited by the power of the electron beam or by photon scattering effects. Importantly, for a given pixel (i.e., output face of an element) much higher electron beam power can be deposited into an element without experiencing the adverse effects of power saturation, thermal loading, and beam focusing. Hence, much higher brightness for a given resolution can be attained.
However, scanning the array of rods with the e-beam causes the light spot to scan in only one dimension. To effect two dimensional scanning, the light spot has to be coupled to some form of optical deflection apparatus (e.g., rotating mirrors). Such apparatus is suitable for many applications (e.g., nonimpact printers, projection CRT), but, nevertheless, utilizes mechanical apparatus to scan in one of the dimensions. Electronic scanning in both dimensions would be simpler and more reliable.